The assailant then vanished - but those who thought they saw him described a tall, thin man (or sometimes a woman) dressed in black clothing, sometimes carrying a special gun that was used in agriculture to spray pesticides on to crops.

​Highly toxic

Victims of the Mad Gasser became severely ill after the attack on their home, with symptoms including full or partial paralysis, vomiting, swelling of the face and a burning sensation in the mouth and on the lips.

​​But who was the Gasser - or Gassers? Theories ranged from an agent carrying out secret government tests with new chemical weapons, to mad scientists, aliens or simply local kids bent on terrorising the locals.

It'd happened before...

It’s worth noting that Mattoon’s Mad Gasser is not the only case of Americans being assailed by a noxious visitor; very similar cases were reported in the mid-1930s in Florida and Virginia. In once case, that of Alice Huffman, the gas proved almost deadly, swelling her throat to the point where she had to be resuscitated and later suffered several convulsions.

The type of gas used in both the 1930s and 1940s attacks was never identified.

Mass hysteria?

However, instead of being down to an assailant or assailants, the authorities - including the FBI - had another explanation as to what was causing the phenomenon: mass hysteria.

Perhaps this explanation was settled upon as the figure was proving to be so elusive, with any sightings of the shadowy person being fleeting, and leaving few clues of their identity behind...

One official, Thomas V Wright, commented:

There is no doubt that a gas maniac exists and has made a number of attacks. But many of the reported attacks are nothing more than hysteria. Fear of the gas man is entirely out of proportion to the menace of the relatively harmless gas he is spraying. The whole town is sick with hysteria.​